Researchers Look for Clues to Autism in the Environment

Marina Sarris
Interactive Autism Network at Kennedy Krieger Institute

Date Last Revised:

June 23, 2014

Date Published:

May 30, 2013

What part does the environment play?

Since the autism rate began rising in the 1990s, parents and scientists have wondered about the role of environmental factors in triggering autism spectrum disorders (ASD). In the medical world, the environment can mean anything from a pregnant woman's age, infections she had, the chemicals she used at work, or the air she and her newborn breathed.

Most autism cases – about 90 percent according to some estimates – have no known medical cause.1, 2, 3 Is a mix of genetic and environmental factors at work? If so, which factors, and how much do they contribute?

These questions took on more urgency in recent years when twin studies suggested that unidentified environmental factors play a more important role in autism than previously believed.4, 5 Researchers discovered that fraternal twins, who shared the same environment before birth but have different DNA, were more likely to both have ASD than siblings who weren't twins.

Investigating pollutants and autism

Some researchers have zeroed in on exposures to air pollution at home, and hazardous chemicals on the job. Several studies found an "association" between pollution, or chemical exposures, and autism. In other words, children exposed to higher levels of some pollutants or chemicals before and/or after birth were more likely to develop autism.

Dr. Heather Volk

Such research is in its infancy, and scientists caution that it's too early to say that pollution or chemicals cause autism just because they occur together. Some recent environmental studies are exploratory in nature: scientists are looking for clues that warrant a more intensive look in future studies.

Certain pollutants have captured researchers' attention because they are known to affect the human nervous system and development, and to affect a fetus.

"Looking at the association between air pollution and neurodevelopmental disorders is a relatively new field. Most manuscripts on the subject came out in the last five years," explained researcher Heather E. Volk, Ph.D., assistant professor of preventive medicine in the Division of Environmental Health at University of Southern California.

In a study published this year, Dr. Volk and her team examined the air pollution exposures of 279 children with autism and 245 children with typical development, called controls, in California. The scientists looked at the locations of homes their mothers occupied during pregnancy and the first year of their children's lives. They estimated the amount of air pollution at those addresses, factoring in the homes' proximity to freeways, traffic volumes and weather conditions. They also considered the region's air quality as measured by the U.S. Environmental Protection Agency (EPA).6

The results? The children with autism were three times more likely to have been exposed to higher levels of traffic and regional air pollution in their first year of life than the typical children. Researchers found a moderate relationship between autism and air pollution even when they took into account social, income and other differences between the families they studied.

Dr. Volk explained that diesel exhaust particles and other air pollutants can cause inflammation and "oxidative stress," two conditions that may also be involved in the development of autism. Like most scientific studies, this one acknowledges that other possible factors could have contributed to autism, such as indoor air pollution, lifestyles or nutrition.6

Another study released in 2013 also found that exposure to air pollutants around the time of birth "may increase risk for ASD."7 That research team, led by Andrea L. Roberts, Ph.D., of the Harvard School of Public Health, looked at 325 nurses who had a child with autism and 22,101 nurses who had a child without ASD. The nurses lived in different parts of the United States. The women who lived in areas with the highest level of air pollution – diesel particulates or metals – had twice the risk of having a child with ASD as women who lived in areas with the least air pollution.

Another look at air quality

Those 2013 studies appear to support the findings of earlier research that found a "potential association" between autism and hazardous air pollution outside the homes where mothers lived at the time of their children's births.

That 2006 study looked at 284 children with ASD and 657 typically-developing children born in 1994 in the San Francisco Bay area. Researchers estimated their exposure to hazardous air pollutants based on a 1996 EPA database. Cars, trucks, planes, trains, dry cleaners, gas stations and manufacturing plants all can pump these chemicals into the air.8

Researchers found a "moderately" higher risk of autism among children born in areas with higher estimated air levels of diesel exhaust particles, metals such as mercury, cadmium, nickel, and chlorinated solvents.8

Even rural areas, far from superhighways, may pose risks. A 2007 study in the Central California Valley, and a 2014 study in Northern California, found that pregnant women living closest to farms treated with pesticides had a higher risk of delivering a child who would be diagnosed with ASD.9, 12

Other studies in North Carolina and West Virginia found associations between autism and air pollution in those states.6, 10

Chemicals at work

While those studies looked at pollution where pregnant women lived, a 2013 study examined possible chemical exposures where mothers-to-be worked. "Typically, occupational exposures tend to be higher than [general] environmental exposures," explained Gayle C. Windham, Ph.D., a reproductive epidemiologist with California Department of Public Health who worked on the study.

Dr. Windham said she became interested in parents' occupations when she conducted earlier research on the theory that fathers of children with ASD were more likely to be engineers. (They aren't, according to her study.)

In her latest study, her team used California birth certificate information to determine the parents' occupation at the time of a child's birth. The team wondered if prenatal exposure to chemicals could affect a child's development in a way that may trigger autism.11

The researchers discovered that the mothers of children with autism were twice as likely to have worked in jobs that exposed them to chemicals as the mothers of children who don't have autism. That was particularly true of jobs that brought workers into contact with exhaust and disinfectants.

The "exploratory" study, which involved 284 children with autism and 659 controls, did not determine the mothers' actual occupational exposures. Instead, researchers looked at whether they had a job that was likely to involve certain chemicals, as determined by a doctor certified in occupational medicine.

For examples, bus drivers, flight attendants, mechanics and firefighters were presumed to be exposed to exhaust and combustion fumes. Medical and dental workers were presumed to be exposed to disinfectants. The job exposures of fathers did not seem to affect the likelihood of having a child with ASD.11

The researchers said other factors could have contributed to the results. They called for more studies to delve deeper into the subject. In fact, Dr. Windham said she already is working on a future study that will look at both genetic and environmental factors in the development of autism.

Dr. Volk said she would like to see a different kind of environmental study, one that is "prospective" or forward-looking.

Many studies begin with children who are already diagnosed with autism and then look backward to determine or estimate their exposures, she explained. In a prospective pollution study, however, researchers would monitor air quality at the homes of women while they're pregnant and later check to see if their babies developed autism. That may shed more light on the issue, she said.